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''This sandbox is in use until August 1, 2011 for UMass Chemistry 423. Others please do not edit this page. Thanks!'' Chem423 Team Projects: Understanding Drug Mechanisms

=Rituximab Fab=

Introduction
Rituximab Fab is used as a prescribed drug to stop transplant rejection, to help regulate autoimmune diseases and to fight cancer. Rituximab has become a key proponent in treatment for cancers such as lymphoma and leukemia and has become the most used chimeric (mouse/human) antibody of its kind. Developed in 1986 by Ivor Royston and Howard Birndorf, Rituximab became FDA approved for clinical use in 1997. Since then Rituximab has become the routine treatment for non-hodgkins lymphoma that otherwise proves to be resistant to certain forms of chemotherapy. Recently, in 2010, Rituximab was approved by the European Commission for use in fighting follicular lymphoma. Because of the wide range of effects that Rituximab causes through its interactions with the CD20 protein, many uses for this antibody have been discovered.

Rituximab is part of a family of antibodies called chimaric monochromal antibodies. These types of antibodies are developed from a single type of parent cell and specifically bind to a single host cell. These types of antibodies do not vary between each other and always interact in the same manner with the chosen cell. Each one of these antibodies is a mixture, chimera, of both rat and human cells both of which play the same role in the host organism. Being chimaric allows for testing on mice before implementation on human cells and combination of human antibodies with mice antibodies makes for simple transitioning between lab testing and clinical testing because the known antibodies have been linked together without making large assumptions with no research.

In brief, Rituximab acts on both benign and malignant lymphocytes, or B-cells. B-cells create antibodies when bombarded with foreign antigens. They are produced in the bone-marrow of humans and when acting normally, help the body fight off disease. If these B-cells are over-produced, under-produced, act abnormally, or are dysfunctional, many different diseases may arise as previously listed. Rituximab acts on the CD20 protein that is found on almost all B-cells. this CD20 protein is unchanged throughout the life of the B-cell.

Multiple proposed mechanisms of action may occur when Rituximab interacts with a lymphocyte (B-cell). The first is known as antibody-dependent cell-mediated cytotoxicity (ADCC) which allows Rituximab to trigger natural cell lysing mechanisms through the use of T-cells and other macrophages. In this proposed mechanism, Rituximab performs as a label to begin cell destruction. Upon further investigation, complement-dependent-cytotoxicity (CDC) became another possible mechanism of action. Through this mechanism, complement proteins are called upon to destroy cell membranes causing overall lysis of the cell. These complement proteins are again naturally occurring, yet usually rely on naturally occurring antibodies to be guided towards non-self antigens. Instead, rituximab acts as the antibody guiding these complements specifically to B-cells. The third and final possible mechanism of action occurs through apoptosis, a more direct route to cell death. Upon initial interaction with the B-cell, the cell triggers programmed cell death. Through this process the cell self-destructs and is immediately destroyed. All three of these mechanisms have been looked into with some uncertainty as to which most likely occurs. Scientists have witnessed many results from treatment such as the shedding of CD23, the down regulation of B-cell receptors, and many other effects that both prove and disprove all three proposed mechanisms.

Rituximab has proven to be a very useful antibody in fighting many human disease, through looking closely at how Rituximab may interact with B-cells, further knowledge about its possible uses will result. This article put together by Chemistry Undergraduate Students at the University of Massachusetts Amherst, will analyze and compile evidence of the chemical mechanisms, binding sites, and applications of Rituximab, a highly used drug in medicine.

Structure
Rituximab is made up of four amino acid chain subunits. Two of these are considered major subunits and two are minor subunits. The two major amino acid sequences are made up of 451 amino acids and the two minor chains contain 213 amino acids. This large protein binds to the surface the surface of B-cells and therefore is exposed primarily to the aqueous external environment. To have stable interactions with the environment, the protein is largely polar, composed almost entirely of beta sheets.

The beta sheets formed in the protein’s native structure form two distinct sides of this protein. Each of these sides is made of woven beta strands that form anti-parallel sheets. The binding site of the protein is found within one of these beta-strand sheets.

Rituximab interacts the surface of its cell membrane through a alpha helix region. This is actually done in two places, where alpha helices of non-polar amino acids are found. Each of these is found at the terminal end of a beta sheet, allowing the active site to bind to the substrate elsewhere on the cell surface.

Each of these structures can be seen in the Jmol native structure found to the right. The red and blue beta strands are the major amino acid strands, while the light and dark green strands are the minor amino acid strands. Each of these has an alpha helix structure, which is shaded in a slightly different color to emphasize its position.

Drug Binding Site
Click to View CD20 Binding Site

Rituximab (Rituximab Fab) is a chimeric antibody with human IgG1 used in the therapy of non-Hodgkin’s B cell lymphomas. This antibody targets B cells by binding to the cell-surface receptor, CD20. CD20 (human B-lymphocyte-restricted differentiation antigen, Bp35) is a hydrophobic transmembrane protein with a molecular weight of approximately 35 kD located on pre-B and mature B lymphocytes. Rituximab has a binding affinity for the CD20 antigen of approximately 8.0 nM, which is similar to the parent murine antibody, 2B8. The amino acid residues alanine (170) and proline (172) within the extracellular loop of CD20 are critical for rituximab binding.residues Ala 170 and Pro 172 Binding to CD20 Selection of random libraries yielded 2 distinct peptides binding Rituximab: 1 peptide was homologous to alanine (170)-proline(172), the other was assumed to mimic the same epitope.

Of Rituximab Fab, the Fab (fragment antigen-binding) is the region in which the antibody is bound to the antigen. A heavy and light chain domain located at the terminal end of the monomer shapes the binding site of Rituximab Fab. Each one of these chains (heavy and light) has a constant and variable portion. The constant chain is identical in all antibodies; the variable chain is unique to each specific B-cell antibodies. The heavy chains of Rituximab Fab are made up of approximately 451 amino acids and the light chains by approximately 213 amino acids.

Rituximab can begin complement activation by binding of Clq to the Fc region (region of an antibody that interacts with cell surface receptors) of an antibody when inducing complement-mediated cell lysis. C1q is a 400kDa protein split into 3 subunits. Each subunit consists of Y-shaped triple peptide helices joined at the stem, forming a globular non-helical head at its end. The helical components of the structure contain varied strands of Glycine, proline, isoleucine and/or hydroxylysine. The globular heads of the structure (along with two serine proteases, Clr and Cls) are then responsible for the multivalent attachment of the C1q structure; forming the complex C1 which triggers the compliment cascade that performs cellular lysis.

Additional Effects
Rituximab is used quite frequently to treat dysfunctional leukemias and lymphomas. Rituximab works extremely well to treat these diseases because of the CD20 binding site. This treatment can also lead to an increase in the number of circulating CD20+ B cells. Rituximab has also shown to be effective in the treatment of multiple sclerosis, rheumatoid arthritis, and anemias. However while effectiveness has been proven, there are also concerns about the safety of the treatment. There is current research being conducted in Norway which will research the effectiveness of rituximab to treat chronic fatigue syndrome. Rituximab is also being used to manage the recipients of kidney transplants. All of these treatments are because of the CD20 binding sites.

Credits
Introduction: David Peltier

Structure: Donald Einck

Drug Binding Site: Ethan Leighton

Additional Effects: Chris Coakley

Citations: Chris Coakley All Green Screen Effects: David Peltier